The element consists of high grade quartz or vitreous silica, and are provided with an intermediate layer of aluminium, 0.0001-0.2mm. thick. The assembly is then heated to near the m.pt. of Al, and quartz plates with a surface finish at least equal to that of elements are used to exert a pressure of 5-15N/mm2 on the assembly in vacuo, to press the elements together. In a pref. modification, one quartz element is bonded to a similar element made of translucent fused silica. Useful also in the electrical or electronic industries, or in mfg. appts. The joints obtd. have very high strength, resisting shock and vibration, with long life and high reliability.